Zeolites are molecular sieves having unique lattice frameworks constructed of alumina and silica cages. The Internal Zeolite Association (IZA) assigns each unique framework type a three-letter code, such as MOR, CHA, or BEA. Zeolites have numerous industrial applications, and zeolites of certain frameworks, such as AEI, are known to be effective catalyst for treating combustion exhaust gas in industrial applications including internal combustion engines, gas turbines, coal-fired power plants, and the like. In one example, nitrogen oxides (NOx) in the exhaust gas can be controlled through a so-called selective catalytic reduction (SCR) process whereby NOx compounds in the exhaust gas are contacted with a reducing agent in the presence of a zeolite catalyst.
Synthetic AEI zeolites are produced using a structure directing agent (SDA, SDA cations are referred to as SDA+), also referred to as a “template” or “templating agent”. SDAs are typically complex organic molecules which guide or direct the molecular shape and pattern of the zeolite's framework. Generally, the SDA serves as a mold around which the zeolite crystals form. After the crystals are formed, the SDA is removed from the interior structure of the crystals, leaving a molecularly porous aluminosilicate cage.
In typical synthesis techniques, solid zeolite crystals precipitate from a reaction mixture which contains the framework reactants (e.g., a source of silica and a source of alumina), a source of hydroxide ions (e.g., NaOH), and an SDA. Such synthesis techniques usually take several days (depending on factors such as crystallization temperature) to achieve the desired crystallization. When crystallization is complete, the solid precipitate containing the zeolite crystals is filtered off. Conventional methods for synthesizing zeolite AEI have a relatively poor yield on silica (a key component of the reaction mixture). Accordingly, it would be desirable to increase the relative yield on silica of the synthesis process and certain aspects of this invention satisfy this need amongst others.